1. Field of the Invention
The invention is generally related to a method for processing radiation sensitive polyimide film precursors and, more particularly, to a method of processing which allows for better retention of the sidewall profile of trenches and the like created in is a film of the polyimide precursors during imaging and developing subsequent to the shrinkage which occurs during imidization.
2. Description of the Prior Art
Polyimide materials are used in a wide variety of semiconductor and integrated circuit applications. These materials can serve as insulating layers between active devices and conductive lines, as carrier layers in multilayer structures, as well as fulfill a wide variety of other functions. Of particular importance in the electronics industry are polyimides based on 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride (BDPA) and p-phenylene diamine (PDA). These polyimides, as well as some others, possess several important properties for integrated circuit and semiconductor manufacturing, including low solvent uptake, low thermal coefficient of expansion (TCE), and high glass transition temperature (Tg) (&gt;325.degree. C.).
The properties that make certain polyimides highly desirable for integrated circuit and semiconductor manufacturing (e.g., high Tg, low TCE, and solvent resistance), also make them difficult to process and pattern. In order to reduce processing steps, efforts have been made to directly pattern radiation sensitive polyimide precursors. For example, U.S. Pat. No. 4,670,535 and U.S. Pat. No. 4,778,859 both disclose photopatterning polyimide precursors.
Both positive and negative working radiation sensitive polyimide precursors have been used in prior art semiconductor devices. U.S. Pat. No. 4,877,718 to Moore et al. discloses a positive-working photosensitive polyimide wherein exposure to a pattern of light renders the exposed areas soluble. Moore et al. further discloses that the exposed areas can then be dissolved using a solvent to leave a pattern which can be used directly as an insulator layer in a semiconductor device. U.S. Pat. Nos. 4,820,612 and 4,828,967 to Mase et al. both show applying a negative-working photosensitive polyimide precursor layer to a semiconductor substrate followed by exposing, developing, and curing to create an insulative layer which is used on the substrate.
The chief advantage of using radiation sensitive polyimide precursors (polyamic acid derivatives) for polyimide film formation is that a stencil is created from a single expose/develop procedure which defines a permanent dielectric insulator. The stencil is subsequently filled with metallization for the electrical conductor and signal lines that make up the integrated circuit package, and no secondary materials or secondary etching steps are required.
A significant disadvantage of using radiation sensitive polyimide precursors for polymer film formation is that a tremendous amount of uncontrolled shrinkage occurs when the photocomponents (e.g., solvents, photoreactive groups, etc.) are driven out during the final imidization cure. The final imidization step assures that the final film has all the properties of a good non-radiation sensitive polyimide material.
A number of photoresist processing techniques have been developed which include ultraviolet (uv) exposure steps for patterning or otherwise affecting photosensitive polyimides. For example, U.S. Pat. No. 4,968,581 to Wu et al. discloses a high resolution photosensitive polyimide wherein deep uv and excimer laser exposure is used to create positive and negative tone images. U.S. Pat. No. 4,980,268 to Bartmann et al. discloses negative photosensitive polyimides which contain 1,2-disulfone moieties as the photoinitiator.
In addition, a number of photoresist processing techniques have been developed which include a post-develop UV exposure step designed to stabilize the photoresist (non-polyimide resists) during subsequent processing. Specifically, Matthews et al., SPIE Conference, Optical Microlithography III, Santa Clara, Calif., Mar. 14-15, 1984, disclose the stabilization of single layer and multilayer resist patterns to aluminum etching environments. In particular, the Matthews article discloses a first LJV exposure step for patterning and a second, post-develop, exposure step to intense UV radiation while increasing the wafer temperature from 100.degree. C. to 200.degree. C. The second or post-develop exposure step with temperature increase is said to enable several common positive photoresists to withstand higher temperatures and planar plasma etching with aluminum. In addition, U.S. Pat. No. 5,001,039 to Ogah et al. discloses a method of manufacturing semiconductor devices wherein a photoresist layer is exposed, developed, and then exposed to uv again prior to heat treatment. As explained in Ogah et al., the second uv exposure step crosslinks the surface of the photoresist material and provides a "hardening" property that prevents drooping at the bottom of the photoresist pattern.
The UV exposure techniques described for photoresists are not directly translatable to radiation sensitive polyimides, such as photosensitive polyimides, used as carrier or insulative layers. Resists are usually on the order of a few microns, whereas radiation sensitive polyimide materials are typically used to create films on the order of twenty microns. Moreover, the "hardening" step for photoresists described in the prior art is merely limited to the surface regions of the resist, not the bulk of the film. With radiation sensitive polyimide precursors, temperatures of approximately 400.degree. C. are used to imidize the precursors into a polyimide film. Photoresists are generally not designed to withstand these extreme conditions, even if they are "hardened" as suggested by the prior art. Furthermore, during curing, polyimide materials undergo massive shrinkage on the order of 50% or more, whereas photoresists do not shrink to this magnitude upon heating. Therefore, surface hardening is all that is required for photoresist materials, but it would not be adequate for photosensitive polyimides. Lastly, the surface hardening steps performed with photoresist materials is designed to prevent droop of the photoresist. Specifically, the photoresist tends to melt or sag at higher temperatures and under harsh conditions, and this tends to narrow a trench opening in the photoresist at its bottom. Conversely, radiation sensitive polyimide materials do not "flow" like a photoresist, so closing off at the bottom of a trench is not a problem (e.g., the bottom of a trench is relatively fixed with polyimide materials).